Method and apparatus for measuring particle size at low concentration

ABSTRACT

Described is an improved method for measuring the particle size of ultrasmall particles (111) which are suspended in a fluid. Laser light (121) is scattered by the particles, and the scattered light (122) is received by a light detector (131) which provides an electrical measuring signal (S m ) which is representative of the intensity of the scattered light (122). According to the present invention, signal components with a relatively low characteristic frequency are removed from the electrical measuring signal (S m ), and the particle size is calculated on the basis of the thus corrected measuring signal (S c ), so that also at a very low concentration of the particles (111) reliable results are achieved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

In the art, there is a need for a method for measuring the particle sizeof ultra-small particles which are suspended in a fluid. Moreparticularly, this concerns particles whose size is typically in therange of 1-3000 nm, but the particles can also be smaller than 1 nm.

The particles can be liquid or solid.

Examples of fields of application where the above-mentioned need exists,are:

environmental technology: measurement of aerosols, for instance sootparticles in air, asbestos particles in air.

biology: measurement of, for instance, virus particles in air, pollen inair.

production-technology: measurement of, for instance, dust particles inair in so-called "clean rooms"; production of ultra-fine particles in agas or a liquid (for instance dyes or medicines).

medical analysis: measurement of body fluids, for instance bloodcomposition, and the measurement of deposition of particles in the humanbody, in particular in the lungs.

2. Description of the Related Art

The need mentioned has existed for some time already, and measuringmethods have already been developed to enable such measurements asmentioned to be carried out. An example of such a measuring method,known per se, is photon correlation spectroscopy, hereinafter designatedas PCS. For an extensive description of this measurement technique,reference is made to the professional literature, such as, for instance,the article "Measurement of Aerosols in a Silicon Nitride Flame byOptical Fiber Photon Correlation Spectroscopy" by M. A. van Drunen et alin J. Aerosol Sci, 1994, vol. 25, no. 5, pp. 895-908. More particularly,in Chapter 2 of that article the theory underlying PCS is set out.

As explained in that article, PCS is based on the fact that particlessuspended in a fluid undergo a Brownian movement, with the movementfrequency of the particles being dependent (inter alia) on their size:the smaller the particles, the greater that frequency. A measuringsignal representing that movement frequency can be derived from lightwhich is reflected by the particles, more particularly from thefluctuations in the intensity of that light, which fluctuations, movedover a particular delay time, are correlated with themselves.

During the performance of a measurement, the measuring apparatus only"sees" a relatively small measuring volume, which is to say that onlylight signals from the particles present in that measuring volume areprocessed. In practice, that measuring volume typically has a magnitudeof the order of 10⁻⁶ cm³. The strength of the measuring signal, that is,the intensity of the light received from that measuring volume isdependent, inter alia, on the concentration of the particles and moreparticularly on the number of particles present in the measuring volume:the more particles are present in the measuring volume, the moreparticles contribute to the measuring signal, that is, the greater thatintensity.

A problem presenting itself here is based on the fact that the particleshave a kinetic energy, that is, a velocity dependent on the temperature,as a result of which some particles will leave the measuring volumewhile other particles will enter the measuring volume. As a consequence,the number of particles actually present in the measuring volume at aparticular time will not be constant but fluctuate over time. Thisfluctuation in the number of particles causes a second intensityfluctuation in the measuring signal, which influences the measuringsignal. This effect is negligible in the case of relatively largenumbers of particles because then the fluctuation in the number ofparticles is negligible with respect to the total number of particles.At low concentrations, however, in particular when the number ofparticles in the measuring volume is less than about 200, a noticeableeffect occurs, which is greater when the number of particles in themeasuring volume is smaller. The influence on the measuring result issuch that the measured size of the particles differs from the actualsize; more particularly, the measured size is greater than the actualsize. In consequence, it has been assumed heretofore that PCS is onlyuseful in cases of sufficiently high particle concentrations, as hasbeen noted in Chapter 1 of the above-mentioned article, with referenceto the article "Analysis of a Flowing Aerosol by CorrelationSpectroscopy: Concentration, Aperture, Velocity and Particle SizeEffects" by R. Weber et al in J. Aerosol Sci., 1993, vol. 24, p.485.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to break through this prejudiceand to improve the known PCS method in such a manner that it has goodutility also at very low concentrations and yields reliable, accurateresults.

The present invention is based on the insight that although theabove-mentioned fluctuations in the number of particles and the secondintensity fluctuations in the measuring signal thereby caused arestatistical phenomena, as are the first intensity fluctuations caused bythe Brownian movement, those second fluctuations occur in acharacteristic frequency range which is appreciably lower than thecharacteristic frequency range in which the first intensity fluctuationscaused by the Brownian movement occur. Therefore, according to thepresent invention, it is possible to make a distinction between thesetwo types of fluctuations.

Thus, according to a first aspect of the present invention, thecalculation of the particle size is carried out solely on the basis ofthe first fluctuations.

Further, according to a second aspect of the present invention, a methodis provided for calculating on the basis of the second fluctuations theparticle concentration, more particularly the number of particles in themeasuring volume.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be clarified by the following description ofa preferred embodiment of a method and apparatus according to theinvention, with reference to the drawings, wherein:

FIG. 1 shows a block diagram of an apparatus according to the invention;

FIG. 2 shows a schematic overview of a measuring device used in theapparatus according to the invention;

FIG. 3 shows a graph which is representative of a measuring signalobtained with the measuring device shown in FIG. 2;

FIG. 4 shows a graph similar to FIG. 3 in a situation where the particleconcentration is low;

FIG. 5 is a graphic representation of a few measuring results;

FIGS. 6-9 schematically show a few examples of a correcting deviceaccording to the present invention; and

FIG. 10 schematically shows an example of a self-setting correctingdevice.

DETAILED DESCRIPTION

With reference to FIG. 1, the structure of a measuring system 1according to the invention will presently be explained in outline. Themeasuring system 1 comprises a measuring device 100 with an output 101for supplying an electrical measuring signal S_(m) ; a correcting device200 with an input 201 for receiving the electrical measuring signalS_(m), and an output 202 for supplying a corrected electrical measuringsignal S_(c) ; and a signal processing device 300 with an input 301 forreceiving the corrected electrical measuring signal S_(c).

In the measuring system 1 according to the present invention, themeasuring device 100 and the signal processing device 300 can beconventional devices, as known in this technical field. In aconventional particle size measuring system, the correcting device 200is absent, and the output 101 of the measuring device 100 is connectedto the input 301 of the signal processing device 300. Although thenature and construction of the measuring device 100 and the signalprocessing device 300 do not constitute a subject of the presentinvention, and are known per se to a skilled person in this technicalfield, they will be briefly explained hereinafter with reference toFIGS. 2-4.

The measuring device 100 comprises a measuring chamber 110 accommodatinga fluid having suspended therein the particles 111 to be examined. Themeasuring chamber 110 has walls which are transparent to light. Themeasuring device 100 further comprises a light source 120 for coherentlight, such as, for instance, a laser. A light beam 121 generated by thelaser light source 120 is directed to the measuring chamber 110, withmeans being present (not shown for simplicity's sake) for converging thelaser beam 121 at a particular point within the measuring chamber 110.

The laser light beam 121 is scattered by the particles 111 in themeasuring chamber 110, the extent of the scatter being dependent on thescattering angle α. At a predetermined position, which is generallyadjustable, a photosensitive detector 131 is arranged, which receivesthe light 122 which has been scattered at a predetermined scatteringangle α and provides an electrical signal 132 which is proportional tothe intensity of the received light 122. The electrical signal 132 isfed to an amplifier 133, which supplies the electrical measuring signalS_(m) at its 101 output. It is noted that the detector 131 and theamplifier 133 can be constructed as one whole. Preferably, the detector131 comprises a photomultiplier tube.

The detector 131 is arranged to receive only those light signals thatoriginate from a limited solid angle, which is designated as the fieldof view of the detector 131. The section of the field of view of thedetector 131 and the area of convergence of the laser beam 121 isdesignated as the measuring volume. The intensity of the scattered light122 is dependent inter alia on the number of particles in that measuringvolume.

Each particle in the measuring volume scatters the incoming light 121 ina characteristic way and generates a spatial pattern of scattered light.If at least two particles are in the measuring volume, the scatteredlight patterns caused by those particles will interfere with each other.As a result of the Brownian movement of the particles, the interferencepatterns will vary in a random, statistically determined manner. This ismanifest in the intensity of the scattered light 122 through afluctuation in the intensity, as schematically represented in FIG. 3,where time t is plotted horizontally in arbitrary units (a.u.) and theintensity I is plotted vertically in arbitrary units (a.u.).

These intensity fluctuations are representative of the diffusioncoefficient of the particles in the medium, which in turn is dependentinter alia on the size of the particles the signal processing device 300comprises a suitably programmed computer which, taking into accountparameters such as the temperature, viscosity of the fluid, etc.,calculates from the fluctuations in the signal received at its input 301the diffusion coefficient and/or the particle size and reproduces them,for instance in the form of a graph and/or printed numbers. An exampleof a conventional signal processing device 300 is based on theperformance of an auto-correlation technique, whereby, briefly stated,it is determined on what time scale an averaging of the signal receivedat the input 301 yields a constant value. A small time scale thencorresponds with small particles.

In FIG. 3 it is shown that the fluctuations referred to occur around anaverage level designated A, which level is dependent on the number ofparticles in the measuring volume. The conventional signal processingdevice 300 has proved to give good results if the signal received at itsinput 301 does indeed have the shape represented in FIG. 3, which occursif the concentration of the particles in the fluid is sufficiently high,more particularly if the number of particles in the measuring volume issufficiently large.

In principle, the number of particles in the measuring volume is notconstant, since owing to the kinetic energy of the particles, particleswill leave the measuring volume while other particles will enter themeasuring volume. As mentioned, the intensity of the scattered light 122is dependent inter alia on the number of particles in the measuringvolume, so that the above-mentioned fluctuation in the number ofparticles will cause an intensity fluctuation.

If the number of particles in the measuring volume is sufficientlylarge, this effect is negligible, and the number of particles in themeasuring volume may be regarded as being constant over time. Moreparticularly, in that case the average intensity level A may be regardedas being constant over time, in which case the conventional signalprocessing device 300, as mentioned, yields good results.

However, if the number of particles in the measuring volume is not largeenough, more particularly lower than about 100-200, the effect mentionedis no longer negligible, but manifests itself in a statisticallydetermined fluctuation of the average intensity level A, asschematically illustrated in FIG. 4. In such cases the conventionalsignal processing device 300 can no longer yield good results. This isillustrated in the graph of FIG. 5, which shows the result of anexperiment with latex particles of a size d₀ of 501 nm (on average),suspended in water with a temperature T=298 K and a viscosity η=0.8904cP. The light source 120 used was an Argon Ion laser, and the laserlight beam 121 had a wavelength λ=514.5 nm. The detector 131 wasarranged at a scattering angle α=75°. Plotted along the logarithmichorizontal axis is the average number of particles <M> in the measuringvolume, as determined through calculation. Plotted along the verticalaxis is the ratio between actual particle size d₀ and the calculatedparticle size d_(pcs) calculated by the conventional signal processingdevice 300. Measuring points indicated by a circle (o) correspond withthe results obtained when the signal input 301 of the conventionalsignal processing device 300 was connected to the signal output 101 ofthe measuring device 100. It is clear to see in FIG. 5 that when thenumber of particles in the measuring volume is lower, the value of themeasured or calculated particle size value d_(pcs) deviates more fromthe real value d₀.

As appears from FIG. 4, the above-mentioned fluctuation of the averageintensity level A occurs with a time scale that is greater than the timescale corresponding with fluctuations in the interference patternscaused by the Brownian movement, which is "interpreted" by theconventional signal processing device 300 as a greater particle size.This implies that the conventional measuring system is then no longeruseful for supplying reliable results. From FIG. 5 it appears that inthe example described, the conventional measuring system no longeryields reliable results for values of <M> less than about 100. At valuesof <M> less than about 5 it was even found to be entirely impossible toobtain meaningful measuring results.

A complicating factor in this connection is that the user of theconventional measuring system does not know whether the measuring resultprovided is reliable or not. To be able to make any pronouncements aboutthat, the user should have information about the number of particles inthe measuring volume, that is, about the concentration of the particles,for which purpose a separate measurement with separate measuringapparatus is required.

Surprisingly, it has been found according to the present invention thatit is possible with relatively simple means to correct for the influenceof the fluctuations in the number of particles. It has been found thatin spite of the fact that the particle number fluctuations arestatistically determined, as are the fluctuations in the interferencepatterns caused by the Brownian movement, the particle numberfluctuations typically occur on a time scale with a considerably greatertime constant than do the fluctuations caused by the Brownian movement.According to the present invention, the two contributions are separablethrough relatively simple filtering techniques. To that end, accordingto the present invention, the correcting device 200 is connected betweenthe measuring device 100 and the signal processing device 300 (see FIG.1).

FIG. 6 shows a first embodiment of a correcting device 200 according tothe present invention. In this simple embodiment the correcting device200A comprises a high-pass filter 210 connected between the input 201and the output 202. The high-pass filter 210, of which the slope ispreferably as great as possible, has a suitably chosen crossover pointor cut-off frequency, such that the signal components of relatively lowfrequency, deriving from the particle number fluctuations, are stoppedwhile the signal components of relatively high frequency, deriving fromthe fluctuations caused by the Brownian movement, are passed.

FIG. 7 shows a second embodiment of a correcting device 200B accordingto the present invention. In this second embodiment the correctingdevice 200 comprises a low-pass filter 220, of which an input 221 isconnected to the input 201 of the correcting device 200B, and adifferential amplifier 230, of which a first input 231 is connected toan output 222 of the low-pass filter 220. A second input 232 of thedifferential amplifier 230 is connected to the input 201 of thecorrecting device 200B. An output 233 of the differential amplifier 230is connected to the output 202 of the correcting device 200B. Thelow-pass filter 220 has a suitably chosen crossover point or cut-offfrequency, such that the signal components of relatively low frequency,deriving from the fluctuations in the number of particles, aretransmitted while the signal components of relatively high frequency,deriving from the fluctuations caused by the Brownian movement, areretained, Thus the low-pass filter 220 provides at its output 222 asignal that is representative of the contribution to the measuringsignal S_(m) caused by the fluctuations in the number of particles, andthe differential amplifier 230 removes that contribution from themeasuring signal S_(m).

An advantage of this second embodiment is that the signal provided atthe output 222 of the low-pass filter 220 can be delivered to a secondoutput 203 of the correcting device 200B as a signal which is indicativeof the number of particles in the measuring volume and of theconcentration of the particles in the measuring volume, respectively.Thus, according to the invention it is no longer necessary to provideseparate measuring equipment for measuring the particle concentration,which constitutes an additional advantage of the invention.

FIG. 8 shows a variant of the first embodiment of FIG. 6. Coupledparallel with the high-pass filter 210 is a low-pass filter 220 which,similarly to the low-pass filter 220 of FIG. 7, provides at a secondoutput 203 of the correcting device 200C a signal which is indicative ofthe number of particles in the measuring volume and of the concentrationof the particles in the measuring volume, respectively.

FIG. 9 shows another variant of the first embodiment of FIG. 6, whichincludes a differential amplifier 240, of which a first input 241 isconnected to an output 212 of the high-pass filter 210. A second input242 of the differential amplifier 240 is connected to the input 201 ofthe correcting device 200D. An output 243 of the differential amplifier240 is connected to a second output 203 of the correcting device 200D.The high-pass filter 210 provides at its output 212 a signal which isrepresentative of the contribution to the measuring signal S_(m) causedby the Brownian movement, and the differential amplifier 240 removesthat contribution from the measuring signal S_(m), for providing at thesecond output 203 a signal which is representative of the contributionto the measuring signal S_(m) caused by the particle numberfluctuations, and which is thus indicative of the number of particles inthe measuring volume and of the concentration of the particles in themeasuring volume, respectively.

The effect of the measures proposed by the present invention can beillustrated with the aid of the graph of FIG. 5. Under the sameconditions the experiment described in the foregoing was repeated, butnow the correcting device 200B of FIG. 7 was coupled between themeasuring device 100 and the signal processing device 300. The cut-offfrequency of the low-pass filter 220 was set at 37 Hz, while the lightintensity fluctuations caused by the Brownian movement of the particles111 typically had a frequency of 384.4 Hz. The measuring points thusobtained with the measuring system 1 of FIG. 1 are indicated in FIG. 5with a cross (x). It appears clearly from FIG. 5 that it is possiblewith the apparatus according to the invention to obtain reliablemeasuring results also in the case of very low values for <M>, even whenthe number of particles in the measuring volume is less than 5.

In the foregoing, it has been mentioned that the filters must have asuitably chosen crossover point or cut-off frequency (-3 dB point). Thatcrossover point can be determined, for instance, experimentally, and bemanually set by the user by setting appropriate values for somecomponents of the filters, as will be clear to a skilled person. Such arelatively simple embodiment can be considered adequate in situationswhere the size of the particles will not change significantly.

However, there are situations conceivable where the size of theparticles is not constant. An example of such a situation concerns theinstance where the particles will react with each other, whereby thenumber of particles will decrease while the size of the particles willincrease. An example of such a situation is a sintering process or acoagulation process. In such situations it is preferred that thecorrecting device be self-adjusting. FIG. 10 illustrates an exemplaryembodiment of such a self-adjusting correcting device 200E, which isbased on the simple variant of the correcting device 200A illustrated inFIG. 6. It will be clear to a skilled person, however, that it ispossible analogously to adapt the variants of FIGS. 7-9 to make thecorrecting device self-adjusting or adaptive.

FIG. 10 shows that the characteristic of the filter 210, in particularthe cut-off frequency thereof, is adjustable by changing a value of acomponent thereof. Which component that is, depends on the type offilter chosen, as will be clear to a skilled person. In the exampleshown, the assumption is that that adjustable component is a variableresistance 213, whose slide can be displaced by means of a motor 280,under the control of a control device 260, which can comprise, forinstance, a microprocessor.

The control device 260 receives in real time information about thefrequency characteristic of the signal S_(m). To that end, for instancea fast Fourier analyser 250 is connected to the input 201, whichanalyser 250 is connected to a data input of the control device 260. Onthe basis of the information received at this data input, the controldevice 260 decides at what frequency the crossover point of the filter210 is to be set. From this, the control device 260 determines a settingfor the variable resistance 213.

When setting the variable resistance 213, the control device 260 can setthe position of the slide of the variable resistance 213. The controldevice 260 can calculate that setting position, or look it up in amemory 270, associated with the control device 260, in which memorythere has been priorly stored a table of the relation between thesetting point of the filter 210 and the setting position for the slideof the variable resistance 213. The motor 280 can be a stepping motor.

The control device 260 can also set the resistance of the variableresistance 213 directly. The control device 260 can calculate thatsetting resistance, or look it up in a memory 270, associated with thecontrol device 260, in which memory there has been priorly stored atable of the relation between the setting point of the filter 210 andthe setting resistance of the variable resistance 213. To be able todetermine whether the instantaneous resistance corresponds with theresistance to be set, the variable resistance 213 can be of doubledesign, with the position of the slide of the second variable resistancecorresponding with the position of the slide of the first variableresistance, and with the slide of the second variable resistance beingconnected to a second data input of the control device 260. Forsimplicity's sake, this variant is not separately shown in FIG. 10.

It has been set out in the foregoing that the signal S_(m) coming fromthe measuring device 100 can contain a disturbing signal component witha relatively low characteristic frequency, as a result of whichcalculations of the particle size are disturbed. Such a disturbingsignal component is introduced when the number of particles in themeasuring volume is relatively low, as a result of fluctuations in thatparticle number. In accordance with the present invention, by performingthe calculations of the particle size exclusively on the basis of signalcomponents of sufficiently high frequency, those calculations can bereliably performed at lower concentration than has been possibleheretofore.

However, the invention also provides advantages in cases where such adisturbing signal component with a relatively low characteristicfrequency is introduced into the signal S_(m) coming from the measuringdevice 100 through a different cause. Also in the case of such differentcauses, the calculations would be disturbed, while through the use ofthe present invention the accuracy of the calculations is no longerdependent on such other causes. An example of such a different causeconcerns the situation where owing to a rapid succession of pulsescoming from the detector, these pulses start to overlap partly.

It will be clear to a skilled person that it is possible to change ormodify the represented embodiment of the apparatus according to theinvention without departing from the concept of the invention or thescope of protection as defined in the claims. It is possible, forinstance, to realize the controllability of the transmission function ofthe correcting device 200 in a different way.

Also, it will be clear to a skilled person that the correction of theelectrical measuring signal S_(m) can be carried out with an electronicfilter (hardware-wise), but that such a correction can also be carriedout by means of a suitable arithmetic means (hardware- orsoftware-wise). That arithmetic means can then be, for instance, anarithmetic unit of the signal processing device 300.

What is claimed is:
 1. A method of measuring the particle size of ultra-small particles suspended in a fluid and experiencing Brownian movement, wherein the concentration of said ultra-small particles in said fluid is low, said method comprising the steps of:irradiating a predetermined measuring volume of said ultra-small particles with a coherent beam of light to cause said light to be scattered by said ultra-small particles into a predetermined scattering angle; detecting the intensity of said light scattered by said ultra-small particles in said predetermined scattering angle, wherein substantial variations in said intensity of said light are a function of said Brownian movement and fluctuations in the number of said ultra-small particles in said measuring volume; generating an electrical measuring signal as a function of said intensity of said light scattered by said ultra-small particles; removing from said electrical measuring signal first signal components with a relatively low characteristic frequency due substantially to said fluctuations in said number of said ultra-small particles in said measuring volume to produce a corrected signal representative of said Brownian movement and substantially free of variations due to said fluctuations in said number of said ultra-small particles in said measuring volume; and calculating from said corrected signal the size of said ultra-small particles.
 2. The method of claim 1, wherein said calculating step includes performing an auto-correlation operation on said corrected signal.
 3. The method of claim 1, wherein said removing step includes filtering from said electrical measuring signal said first signal components with said relatively low characteristic frequency, wherein said relatively low characteristic frequency is of a level to include variations caused by said fluctuations in said number of said ultra-small particles in said measuring volume, whereby said corrected signal includes only second signal components with a relatively high characteristic frequency substantially due to said Brownian movement.
 4. The method of claim 1, wherein said removing step includes generating an auxiliary signal having said first signal components with said relatively low characteristic frequency, such that said auxiliary signal includes said first signal components representative of fluctuations in the number of said ultra-small particles in said measuring volume and substantially free of second signal components due to said Brownian movement.
 5. The method of claim 4, wherein said removing step includes subtracting said auxiliary signal from said electrical measuring signal to produce said corrected signal.
 6. The method of claim 4, wherein said calculating step includes calculating from said auxiliary signal the number and concentration of said ultra-small particles in said measuring volume.
 7. A measuring system for measuring the size of ultra-small particles suspended in a fluid at low concentrations and experiencing Brownian movement in a predetermined measuring volume of said fluid, said measuring system comprising:measuring means for generating an electrical measuring signal representative of the intensity of a light beam scattered by said ultra-light particles in said measuring volume along a predetermined scattering direction; correcting means connected to said measuring means for receiving said electrical measuring signal and generating a corrected signal by removing from said electrical measuring signal first signal components of a relatively low characteristic frequency caused by fluctuations in the number of said ultra-small particles in said measuring volume, such that said corrected signal is a function of substantially only said Brownian movement of said ultra-small particles; and signal processing means connected to said correcting means for receiving said corrected signal and calculating therefrom the size of said ultra-small particles.
 8. The measuring system of claim 7, wherein said correcting means comprises a high-pass filter having an input connected to said measuring means and an output connected to said signal processing means.
 9. The measuring system of claim 8, further including a low-pass filter having an input connected to said measuring means and an output connected to said signal processing means.
 10. The measuring system of claim 7, wherein said correcting means includes a programmable arithmetic means for filtering from said electrical measuring signal said first signal components caused by said fluctuations in the number of said ultra-small particles in said measuring volume.
 11. A measuring system for measuring the size of ultra-small particles experiencing Brownian movement in a predetermined measuring volume comprising:measuring means for generating an electrical measuring signal representative of the intensity of a light beam scattered by said ultra-light particles in a predetermined scattering direction; correcting means connected to said measuring means for receiving said electrical measuring signal and generating a corrected signal by removing from said electrical measuring signal low-frequency components caused by fluctuations in the number of said ultra-small particles in said measuring volume, such that said corrected signal is substantially a function of said Brownian movement of said ultra-small particles, said correcting means comprising a low-pass filter having an input connected to said measuring means and an output, and a differential amplifier having a first input connected to said output of said low-pass filter, a second input connected to said measuring means and an output; and signal processing means connected to said output of said differential amplifier for receiving said corrected signal and calculating therefrom the size of said ultra-small particles.
 12. The measuring system of claim 11, wherein said output of said low-pass filter connects to said signal processing means.
 13. A measuring system for measuring the size of ultra-small particles experiencing Brownian movement in a predetermined measuring volume comprising:measuring means for generating an electrical measuring signal representative of the intensity of a light beam scattered by said ultra-light particles in a predetermined scattering direction; correcting means connected to said measuring means for receiving said electrical measuring signal and generating a corrected signal by removing from said electrical measuring signal low-frequency components caused by fluctuations in the number of said ultra-small particles in said measuring volume, such that said corrected signal is substantially a function of said Brownian movement of said ultra-small particles, said correcting means comprising a high-pass filter having an input connected to said measuring means and an output, and a differential amplifier having a first input connected to said output of said high-pass filter, a second input connected to said measuring means, and an output; and signal processing means connected to said output of said differential amplifier and to said output of said high-pass filter for receiving said corrected signal and calculating therefrom the size of said ultra-small particles.
 14. A measuring system for measuring the particle size of ultra-small particles suspended in a fluid at low concentrations and experiencing Brownian movement comprising:a measuring chamber containing said fluid with said ultra-small particles suspended therein; a light source means for directing a beam of coherent light at said measuring chamber to illuminate a measuring volume of said fluid; a light detector disposed at an angle with respect to said beam of coherent light to detect light scattered by said ultra-small particles; a correcting device connected to said light detector, said correcting device comprising signal removal means for receiving from said light detector an electrical measuring signal of the intensity of said scattered light and removing from said electrical measuring signal first signal components with a relatively low characteristic frequency to produce a corrected signal substantially representative of said Brownian movement only and substantially free of variations due to fluctuations in the number of said ultra-small particles in said measuring volume; and a signal processor connected to said correcting device, said signal processor having means for calculating the size of said ultra-small particles.
 15. The measuring system of claim 14, wherein said correcting device comprises a programmable arithmetic unit. 